Semiconductor gas sensor

ABSTRACT

A semiconductor gas sensor for use in equipment for detecting small amounts of H 2  S. The method of sensor fabrication comprises spray deposition of a mixture of metal oxides mixed together with various metal and non-metal materials which serve in the finished product as activators, dopants, and/or film binder materials, and including in suspension a molecular sieve material, for enhancing and defining porosity on a scale of molecular dimensions in the finished sensor. All of the foregoing materials are suspended in a suitable solution and preferably sprayed onto a heated insulating substrate to form the finished product. The example sensor, capable of selective detection of H 2  S in air and a sensitivity of less than 1 PPM (part per million), is comprised of a platinum activated alumina, tin oxide, and zeolite molecular sieve material.

FIELD OF THE INVENTION

This invention relates to semiconductor gas sensors and to methods offabrication thereof, and more particularly to a unique spray depositionmethod wherein an improved semiconductor sensor is fabricated whichcomprises preselected gas sensor components in combination with amolecular sieve material to enhance and define porosity in the finalsemiconducting film. A specific example of a sensor and its method offabrication is described which is capable of a selective detection of H₂S by changes in the conductivity of the sensor relative to theconcentration of H₂ S in the gas sample.

BACKGROUND OF THE INVENTION

Various semiconducting metal oxides have been used in conjunction with avariety of metal and non-metal additives in the fabrication of gassensitive films suitable for use in gas detection apparatus. Exposure ofsuch gas sensitive films to the gas of interest generally is detected asa change in conductivity of the film. In general, these prior devicesexhibited inherent deficiencies in sensitivity, selectivity, responseand recovery times, and/or calibration stability. The electricalcharacteristics and subsequent gas response characteristics of suchmaterials when employed as gas sensors in previous gas sensing equipmenthave been found to be highly dependent upon film properties such asthickness, uniformity of composition, purity, film porosity, anddensity. Since it has previously been difficult to adequately controlthe foregoing factors this art has been seeking a technique offabrication which would be capable of producing films with the abovementioned and other properties well controlled. In addition it is ofcourse desireable that any new technique should be reproducible and costeffective. Further, the previous sensors were sometimes of limitedutility if they were not capable of low temperature operation. Thisproperty is advantageous when sensing flammable gases in that therewould be a reduced hazard of flammable gas ignition by the operatingsensor, as well as an increased realiability and sensor life, reducedsensor power requirements, and better compatibility with on-chipintegrated signal processing circuitry.

The previous attempts to achieve the foregoing properties employedseveral deposition techniques for depositing the materials andcombinations of materials found suitable for use in semiconducting gassensors. Typically the fabrication methods employed have includedsintering, vacuum evaporation, sputtering, chemical vapor deposition,pyrolytic spray deposition, and solution coating. Besides the previouslymentioned drawbacks, each of the foregoing methods creates specificproblems. For example, sintered films often lack sensitivity due to lackof porosity in the processed material. Vacuum evaporation, sputtering,and chemical vapor deposition processes are costly, and sometimes lackflexibility by making it difficult to properly control the introductionof certain dopants.

In practice, the spray pyrolysis techniques consist of spraying asolution containing a soluble salt of the cation of interest with theaid of a carrier gas, onto a heated substrate whereupon the solutionundergoes a chemical reaction to form the resultant film. This processis characterized by relatively high substrate temperatures duringdeposition; e.g. several hundred degrees Centigrade. The lower limit ofsubstrate temperature is dictated by the required chemical dissociationreaction. To be successful there must be complete dissociation of thesalt and this reaction rate therefore imposes a limitation on thedeposition rate.

During film formation, film uniformity can be critically influenced byspray turbulence, lateral gas flow across the substrate and boundarylayer formation in the vicinity of the substrate itself. Substratetemperature control is also very critical for film uniformity. Care mustbe taken to minimize thermal shock which accompanies the spraying of thematerial onto the heated substrate. Other deposition parameters havealso required close control. The carrier flow rate affects the size andvelocity distribution of droplets in the spray which affects thedynamics of impingement. These and other factors inherent in thisprocess have resulted in increased process complexity and cost.Additionally, resultant films produced by this process are generallycharacterized by the presence of large grain sizes which results in lowresistivity which further limits their usefulness for gas sensingapplications. The process is also limited in its application to onlythose materials which can undergo the appropriate dissociation reaction,to produce the desired product on the heated substrate.

Solution coating techniques are more widely used for gas sensorfabrication because of the simplicity of the process and suitability ofthe film properties. Small grain size films of high porosity arepossible to achieve. A solution containing the materials in suspensionand/or in the form of a soluble salt is applied by brush or dipping to asuitable heated substrate where at a temperature of typically 100degrees C. to 200 degrees C. the volatile components are driven off. Theresultant substrate and film are then partially sintered by firing at ahigher temperature typically 600 degrees C. to 800 degrees C. Enhancedfilm porosity is often achieved by addition of materials whichvolatilize and evaporate from the film during high temperature firing.Examples of such materials are starch, wax, stearic acid, and silicagel.

The solution coating process is operator intensive and techniquesensitive. For these reasons solution coating is not suitable for batchprocessing and does not produce uniform product. For example, filmthickness, grain size, chemical composition uniformity and porosity, allcan vary which results in non-uniform gas sensing properties within thefilm and from sensor to sensor. Further, certain desireable additives,particularly transition metals such as platinum or palladium introducedto the source solution as soluble organometallic salts, have thetendency to precipitate out of or localize within a static solution,presumably because of solubility limits or due to chemical reaction andare therefore difficult to use when attempting to make a uniformproduct. Additionally, contamination in the final film by the anion ofthe soluble salt is an undesired result in both solution coating andpyrolytic spray processes.

Sensor materials which have been used in the past for gas sensitivefilms include a number of semiconducting metal oxides, such as SnO₂,ZnO, Fe₂ O₃, Al₂ O₃, Ga₂ O₃, and In₂ O₃. Examples of combinations ofthese materials with other materials for specific gas sensingapplications are presented in the patent issued to Barry Bott Feb. 11,1975, U.S. Pat. No. 3,865,550. The theory of operation generallyproposed for these materials involves an electrochemical reaction of thegas with the solid surface of the heated sensor. The result of thisreaction is to produce a charge transfer wherein an increase or decreasein the number of mobile carriers in the material takes place. In such away, the conductivity in surface layers and at intergrannular contactsin the film is changed. This is measured usually as a change inconductance proportional to the gas concentration. Accordingly, it ishighly desirable for sensors to have a large film surface area to volumeratio in order to exhibit the requisite sensitivity. While previoussensors with a high degree of porosity have had high sensitivity, suchdevices inherently are not selective and require operation at elevatedtemperatures usually above 250 degrees C. to preserve acceptableresponse and recovery times.

Boardman, Jr. et al. U.S. Pat. No. 3,901,067 issued Aug. 26, 1975, havedescribed a sensor for selective detection of H₂ S in air. This sensorhas been shown to operate selectively at somewhat lower temperature andwithin a narrow range. Other similar thin film sensors are commerciallyavailable. Such sensors generally suffer from poor response and recoverytimes, generally on the order of several minutes. When operated athigher temperatures, sensors of this type become electrically unstable,lose selectivity, and are short-lived.

None of the above sensors combine desireable characteristics of fastresponse and recovery times with selectivity and stability at lowoperating temperatures. Response times of a few seconds and comparablerecovery times are necessary for most gases, particularly H₂ S becauseof its toxicity.

Where fast response and recovery times are achieved at higher operatingtemperatures (above the temperatures where thin film sensors usuallyoperate), film stability considerations become more important. Thickfilm sensors fabricated by solution coating processes generally satisfystability requirements. Sensitivity in these films is enhanced by a highdegree of film porosity. However, selectivity has previously beenlacking in thick film sensors.

OBJECTS OF THE INVENTION

It is therefore an object of the present invention to provide a processfor fabricating semiconductor gas sensors that is low cost yet flexiblein nature and which readily lends itself to batch processing.

It is yet another object of the present invention to provide a processfor fabricating gas sensors which results in reproducible uniform filmswhich comprise substantially the components of the original mixture intheir same weight percent, uniformly distributed.

It is yet another object of the present invention to provide arelatively low temperature deposition process for gas sensorfabrication.

It is still another object of the present invention to provide a spraydeposition process where no chemical reaction occurs during depositionon the substrate.

Further, it is an object of the present invention to provide a highsensitivity semiconducting gas sensor article fabricated by the processwhich is capable of selective detection of H₂ S in air with relativelyfast response and recovery times and which operates with low heaterpower consumption.

SUMMARY OF THE INVENTION

It has been discovered that the above and further objects and advantagescan be achieved when preselected gas sensor semiconductor materials aresuspended in a carrier solution and spray deposited onto a heatedinsulating substrate which solution also contains a suspension ofmolecular sieve material in combination with the semiconductingmaterials. Preferably other materials are also suspended in the solutionwhich act as activators, dopants and/or film binder materials in thefinished sensor product. Selectivity of the finished sensor is enhancedwhen molecular sieve materials are incorporated into the final structureof the sensor. Zeolites, alumino-silicate structures whose moleculararrangement is such as to result in a porous structure with pores ofmolecular dimensions are typically used as molecular sieve materials. Arange of zeolite materials with different pore sizes are commerciallyavailable. A given zeolite will have a discrete pore size and a verystable structure. The selectivity enhancement in semiconductor gassensors incorporating these materials is not fully understood andwithout being bound by any specific theory, but it is believed that thesieve material acts in the finished article to define and enhanceporosity in the finished films. Large gas molecules which otherwisemight act as interferences may by virtue of the zeolite structure beeffectively screened from the sensor materials, where as the smaller H₂S molecule can enter the bulk of the solid sensor and contact thesemiconductor sensor material. Further, the zeolites themselves areknown to exhibit catalytic properties which may enhance the electrochemical reactions required for the conductivity change which ischaracteristic of the operation of a semiconductor sensor.

The semiconductor films of this invention prepared as describedhereinafter therefore exhibit a high degree of defined porosity in astructure having good mechanical properties which will operate atrelatively low temperatures with good selectivity and sensitivitytogether with fast response and recovery properties.

The spray deposition process of the present invention comprises sprayinga solution of the herein described constituents suspended in powder formin an organic solvent or water or a mixture of the two. No chemicalreaction at the substrate occurs, other than evaporation of the carriersolvent. The use of a low substrate temperature without the requirementof a chemical reaction, results in reduced susceptibility of the processto the control problems inherent in the pyrolytic spray process thussignificantly reducing the process complexity and cost. The process ofthe present invention is flexible in that it is readily adapted todeposition of many materials combinations. Film reproducibility anduniformity are significantly enhanced compared with films produced bysolution coating, and the present process permits incorporation ofzeolites into the film which have been found to be highly desireable forproducing sensors of improved selectivity and sensitivity. Further,single or multiple layered films of controlled thickness and desiredporosity can now be reproducibly fabricated.

In the inventive process, the source material is prepared by mixing oneor more metal-oxides in powder form with a metal or non-metal dopantand/or film binder and molecular sieve. The metal-oxide would be chosenfrom the many of those which have been demonstrated to possess gasresponsive properties. The exact choice of metal-oxide to be used wouldbe based on the gas to be sensed. Metal dopants are usually selectedfrom among the transition metal group and act as activators or catalystsin promoting gas-solid electrochemical reactions. Film binding materialssuch as ceramic materials, have been incorporated in such sensors toimprove film strength after post deposition anneal. Zeolite molecularsieve materials are commercially available with pore sizes ranging fromabout three angstroms to ten angstroms. The choice of zeolite would bebased on the gas to be sensed. Preferably for sensing H₂ S, five (5)angstrom material is used.

The materials selected are sized and mixed to the desired constituencyusually by weight percent. Before weighing and mixing it is desireableto dehydration bake the powders for about 120 minutes at 115 degrees C.Heating during thorough mixing may also be necessary to minimizeabsorbed moisture related packing or clumping.

All or part of the resultant mixture is added to a liquid such as anorganic solvent or water or any liquid or mixture of liquids that iscapable of sustaining a solids in liquid suspension, so that the mixedpowder is suspended in a carrier liquid that is suitable for spraydeposition. Heating and/or stirring may be employed to assure uniformsuspension of the material. In addition, suspension promoting agents maybe added that do not deleteriously affect the final properties of thesensor.

Suitable substrates are positioned in the field of deposition and heatedby either conduction or radiant heating. The lower limit of temperaturemust be high enough to accomplish the desired evaporation orgasification rate of the carrier liquid of the impinging solution andtypically the preferred temperature employed will be in the range of 50degrees C. to 150 degrees C. Any suitable carrier gas can be employedand air or nitrogen are preferred. The film deposition rate iscontrolled by the degree of dilution of the source solution, sourcesolution flow rate, carrier gas velocity, and the physical configurationof the apparatus with its spray nozzle to substrate spacing. For a givendeposition rate, the film thickness is determined by the duration of thedeposition. Film thickness can be measured by interference techniques orany of a number of well known techniques. After deposition is complete,the substrates are removed and annealed at an elevated temperatureusually in air to remove organic solvents and strengthen the film.

BRIEF DESCRIPTION OF THE DRAWING

An illustrative and presently preferred embodiment of the invention isshown in the accompanying drawing in which:

FIG. 1 is a diagrammatic illustration of a spraying apparatus used inthe spray deposition of semiconductor films for gas sensors.

FIG. 2 is an illustration of a sensor comprising an interdigitatedelectrode structure disposed on a ceramic substrate and coated with asprayed semiconductor film wherein a change in film electricalconductance upon exposure to gas can be measured.

DETAILED DESCRIPTION OF THE DRAWING

Reference should be made to FIG. 1 which is a diagrammatic illustrationof a spraying apparatus useful in the spray deposition of semiconductorfilms for gas sensors according to this invention. The system comprisesa spray nozzle 1 wherein a suitable carrier gas and the source solutionmix and are sprayed so as to form a cloud of droplets of the solution,with a preselected velocity and spatial pattern, designateddiagrammatically at 3. The source solution is contained in a bottle 2and is fed directly to the nozzle 1. The source bottle includes meansfor stirring such as a conventional stirring bar 4 and is placed on aheater and stirrer 5 to maintain uniformity within the solution. Thespraying target 10 on support 11 is heated either directly by, forexample, a regulated hot plate or indirectly with infrared lamp 13.Substrates placed in the target position and within the deposition fieldare heated to between 50 degrees C. and 150 degrees C. to afford rapidevaporation of the solvent after impingement and so as to preventwetting of the substrate during deposition. Two shutters are shown inthe figure. The primary shutter 6 is utilized to directly shield thesubstrates from the spray stream, and may be used to visually establishthe pattern of the deposition field and alignment of substrates ifnecessary, and allow the spraying system to stabilize both dynamicallyand thermally before beginning the deposition. A remote shutter 8 may beused for the same purposes. To minimize gas turbulance in the systemcaused primarily by exhaust flow, a cylindrical air baffle 7 is usedwhich aids in stabilizing the spray stream. The flow rate of the carriergas is monitored by a flow meter 9 and regulated by a low pressureregulator 12. The spray deposition is contained within a clear plasticenclosure 15 which is vented by vent duct 16 which is isolated from thesystem by a conventional filter to minimize circular air turbulance inthe area of the substrates, a shelf 13 isolates the upper part of thesystem from the lower part and helps provide support structure for theair baffle 7 and the shutters 6 and 8. An additional support 20 iscontained in the container 15 for supporting the nozzle 1.

The source bottle 2 is graduated to aid in calibration and control offilm thickness. All or part of the mixture solution may be sprayed.Varying the pressure within the source bottle by means of regulator 22connected to a source of gas under pressure will alter the depositionrate. The deposition rate is controlled by any suitable means ofcontrolling the solution flow rate and ultimately the mixing rate in thenozzle 1. The carrier gas can be any suitable gas such as air ornitrogen. Because the deposition process involves the evaporation of anorganic solvent within the vented enclosure 15, certain flammabilityhazards may exist which may be minimized in an inert carrier gas. Thecarrier gas flow rate determines primarily the velocity and spatialdistribution as well as the droplet size in the spray.

The described apparatus is illustrative only and modifications of thissystem to make the process continuous can be straight forwardlyaccomplished. For example, a modified apparatus could include a beltdriven substrate track to increase throughput. A belt driven systemwould simplify loading of substrates which then could be external to thesystem. Additionally, a belt driven system could conceivably utilizeseveral spraying steps in series as an in-line process for fabricatingcomposite films.

After spray deposition is complete, the substrates are removed from thesystem and placed in a conventional annealing furnace. An annealing stepat high temperature, usually several hundred degrees Centigrade andbased on 1/3 to 2/3 of the temperature of the melting point of themetal-oxide, serves to remove any organic solvent which may beincorporated in the film, and serves to strengthen the film in terms ofmechanical properties.

FIG. 2 illustrates an example of the substrate which can be used in theexample of the present invention. The substrate is a ceramic base 25with Platinum interdigitated electrodes 26 disposed on one side and aPlatinum meandor pattern laser trimmed and used as a heating element 27disposed on the opposite side. In operation, between 150 to 175 mA ofelectrical current is passed through the heater element which heats thesensor to between about 150 degrees C. to 200 degrees C. Platinum leads28, 29, 30, 31 are attached providing electrical connection to theheater 30, 31 and interdigitated array 28, 29. A mechanical connectionis reinforced by a ceramic coating (not shown) and the heater element ispassivated for protection. The substrate is available from RosemountEngineering Ltd. Platline Div., Sussex England.

Materials combinations useful in semiconductor gas sensors, particularlymetal oxides in powder form such as SnO₂, ZnO, Al₂ O₃, Ga₂ O₃, FE₂ O₃,In₂ O₃ or mixtures thereof which are usually mixed with metal andnon-metal dopants or activators, and or binding materials and, as apreferred embodiment of the present invention, zeolite molecular sievematerials useful as agents for defining and/or enhancing porosity, aredirectly compatible with device fabrication by the process of thepresent invention.

EXAMPLE 1

A semi-conductor gas sensor was fabricated according to the process ofthe present invention which possessed less than 1 PPM sensitivity andrelative selectivity for H₂ S gas in air with rapid response time, ofthe order of several seconds, and comparable recovery times. The sourcematerial used was a mixture containing approximately 70% by weight tinoxide with 24% by weight alumina activated with 1% Pt (available fromAlfa Ventron Div. Thiokol Corp.) and 6.0% zeolite (available from LindeDiv. Union Carbide Corp.). The powders are dehydration baked at 115degrees C. for 120 minutes before weighing, then thoroughly mixed usingstandard laboratory procedures. The mixture is suspended in an organicsolvent such as isopropyl alcohol and/or water or a mixture of the twoforming a dilute solution of about 500:1 by volume liquid to solidmixture.

The mixture suspended in solution was sprayed using a spray nozzle model#1/4JCO-SS obtained from Spraying Systems, Inc. North Ave. Wheaton, IL,at a carrier gas of nitrogen flow rate of about 3.0 liters/minute forproper spray velocity and spatial distribution to achieve a uniformdeposition on the substrate. A circular deposition field was establishedwith acceptable thickness uniformity over an area of about 30 mm dia. ata distance of about 30 cm from the nozzle. Such a deposition field canreadily accomodate up to about 100, 2.5 mm by 2.5 mm substrates in oneprocess deposition step. The substrates were heated prior to spraying toabout 50 degrees C. and maintained at that temperature during depositionto assure rapid evaporation of the liquid. Under these conditionsdroplet impingement on the substrate was seen visually as aninstantaneous event of droplet wetting of the substrate surface andevaporation. The dynamics of the process are stabilized as to preventwidespread wetting, accumulation, or puddling of the solution on thesubstrate.

A typical film thickness resulting was several hundred microns inthickness and was obtained by spraying for twenty to thirty minutes.

After spray deposition is complete, the film was annealed by heating at700 degrees C. in air for twelve hours to remove residual solvents andstrengthen the film. After annealing, film mechanical properties andadherance were satisfactory.

After completion of the sensor it can be used in a conventional mannerin electronic equipment designed for relating changes in conductivity tospecific gas concentrations. Such equipment normally employes circuitryfor applying current and voltage to the substrate heater while measuringsemiconductor conductance with conventional circuitry and covertingconductance to a gas concentration. As with other semiconductor gassensors, conduction electrons in semiconducting metal-oxides such as tinoxide originate from shallow donor levels near the conduction band edgeand from the release of trapped charge at surface acceptor states.Plotting electrical conductivity in air (OHM⁻¹ CM⁻¹) versus inverseabsolute temperature (°K.⁻¹) results in a curve where portions areapproximately fitted by an expression of the form σ=A exp [-E_(A) /kT]where A is a material constant, T is absolute temperature, k isBoltzmann's constant and E_(A) is the activation energy for conductionelectrons in the material. The temperature regions of these curvescorresponding to the region of operation of these sensors is fitted withvalues for E_(A) ranging from 0.7 eV to 1.0 eV. These high activationenergies have been attributed to thermal emission of trapped electronsfrom ionized surface acceptor states, thought to be ionized oxygenatoms. The predominant low temperature form of ionized oxygen is O₂ ⁻.At higher temperatures above about 200 degrees C., O⁻ and O⁻⁻predominate. The presence of certain materials such as Pt reduces theE_(A) of the surface acceptor levels.

The reaction involving H₂ S is believed to be described as follows. H₂ Sgas reacts with chemically adsorbed ionized oxygen forming volatileproducts and freeing trapped electrons from the oxygen back to thesolid. The result is conductivity modulation in the surface layers andat intergrannular contacts of the solid measured as an increase inconductivity.

For oxygen, the adsorption process is described as ##EQU1## where k_(o)is the reaction rate constant for chemisorption, and e- is an electronfrom the solid.

Physically adsorbed H₂ S gas can move across the surface until itencounters an O₂ ⁻ _(ads) (Langmuir process), or H_(s) S gas candirectly interact with O₂ ⁻ _(ads) (Eley-Rideal process). The pertinantreaction is believed to be ##EQU2##

The rate constant for the reaction is ##EQU3##

The [] refer to concentration. For small concentrations of H_(s) S, [O₂⁻ ] can be considered a constant. Additionally,

    [H.sub.2 ].sup.2 =[SO].sup.2 =[e-]

Therefore,

    [e-]=k'.sup.[H.spsb.2.sup.S].spsp.2/3

We also have that the film conductivity, and thus its measuredelectrical conductance, G, are proportional to [e-].

G=a[H₂ S]^(b) is the expected dependence of conductance on H₂ S gasconcentration in air with b=2/3. This is substantially the formobserved. In actuality we have found that the exponent, b, andcoefficient, a, can both be controlled to some extent by changes in filmthickness, grain size and porosity, and dopant concentrations. Theobserved trends are not unexpected and suggest optimization of filmproperties for a given desired response characteristic. Following theforegoing analysis the process of the present invention can be adjustedto achieve this optimization.

Sensors fabricated in the herein described manner have been shown to bevery well suited to detection of H₂ S gas in air with lower limit ofsensitivity below 1 PPM. The sensitivity range of particular interest inhealth and safety monitoring and protection instrumentation is usually0-50 PPM. In addition these sensors are not degraded by highconcentrations of H₂ S. Complete recovery after exposures as great as 1%H₂ S in air has been demonstrated. The response time for a typicalsensor to 10 PPM H₂ S in air is less than 10 seconds to reach 90% offinal value. The final value of resistance for 10 PPM H₂ S in airtypically represents about 500× decrease in film resistance.

Interference data has been collected for several common interferinggases. These gases are SO₂, CO, H₂, Hexane, Methane, and aromatedPropane. The sensor response to these gases at relatively highconcentration is less than an equivalent response to 1 PPM H₂ S.Additionally, the sensor response to changes in relative humidity isless than an equivalent response to 1 PPM H₂ S. The selectivity isattributed in part to the relatively low temperature of operation andthe presence of zeolite in the film.

What is claimed is:
 1. A semiconductor sensor for specific gasescomprising a multiple layered structure including an insulatingsubstrate having a discrete semiconductor layer deposited thereon formedby spraying a liquid suspension of a mixture comprising a preselectedsolid metal/oxide semiconductor capable of sensing a specific gas and aporosity enhancing and defining agent capable of defining pore sizes insaid layer of a predetermined size which is selected to permit apredetermined specific gas to come into contact with the semiconductorwithin said layer.
 2. The article of claim 1 wherein said mixture ofsolids in addition contains a transition metal activator for the metaloxide semiconductor.
 3. The article of claim 2 wherein said insulatingsubstrate has deposited thereon at least two electrical contacts betweenthe substrate and the semiconductor containing layer, said contactsbeing separated from each other and capable of being electricallyconnected through said semiconductor layer.
 4. The article of claim 3wherein said insulating substrate contains a resistance heating elementincorporated therein which is physically separated from said electricalcontacts and said semiconductor layer and capable of heating saidsubstrate to a temperature in excess of 150° C.
 5. The article of claim1 wherein the porosity increasing and defining agent includes zeolite todefine a predetermined pore size.
 6. The article of claim 5 wherein atleast 70% by weight of the semiconductor gas sensor layer is a gassensing metal oxide.
 7. The article of claim 6 wherein the metal-oxideis selected from the group of metal oxides including stannic oxide, zincoxide, aluminum oxide, gallium oxide, ferric oxide, indium oxide, ormixtures thereof.